The present invention is particularly applicable to an inductive output tetrode device (IOT) such as a KLYSTRODE (Registered Trade Mark, Varian Associates Inc). The advantages of inductive output tetrode devices (hereinafter referred to as "IOTs") are well known but previously proposed designs have suffered from problems in that it has been necessary to provide a number of tubes, each of which may be required to be used with a number of different cavities in order to provide the instantaneous bandwidth required (e.g. 8 MHz) over the entire television frequency range (e.g. 470-860 MHz). In klystrons, this requirement is currently met by stagger tuning of the various cavities included along the electron beam path to give outputs at different frequencies which add to provide the required bandwidth. However, this is not possible with conventional IOT design.
Another problem which has been encountered results from the high voltages on the order of 30 kV, which the cathode and grid must maintain, especially since the input cavity may define an external part of the IOT and therefore might be handled in normal usage. The present invention arose from an attempt to provide a system which obviates or mitigates some or all of the problems associated with maintaining high cathode and grid voltages while providing an IOT suitable for television applications.